Electrolytic polishing of metal



Patented Sept. 21, 1945 EECTROLYTIC POLISHING METAL Robert W. Manuel, Chicago, 111., assignor to Crane 00., Chicago, 111., a corporation of Illinois No Drawing. Application September 8, 1939, Serial No. 293,885

1 Claim. (Cl. 204-140) This invention relates to'novel methods of obtaining polished or mirror-like surfaces on metals such as, for example, zinc, copper, brass and the like. More particularly, it relates to an electrolytic method of treating such metals whereby the surfaces are polished without the necessity of resorting to mechanical bufllng or polishing processes.

Methods heretofore devised for the electrolytic polishing of metals have been useful principally for the treatment of relatively small flat surfaces such as are used in the microscopic study of metals and metal alloys. The known processes, of which I am aware, are too difflcult to control to be of practical value in the large scale or volume polishing of irreguly shaped articles which are to'be plated with nickel, chromium or the like metal, or other protective coating. One weakness prevalent in some of the known processes is that the electrolytic solution is a rather poor electrical conductor, thus the current distribution over an irregular surface is objectionably uneven. The current density necessary to produce a mirror-like surface is very critical, thus if the current distribution over the whole of the surface of the article to be polished is not maintained relatively even, the polishing effect also will not be even.

In the latter connection, for example, the

French Patent No. 707,526, Societe le Material Telephonique, 1930, discloses a process which may be applied to'the polishing of brass in a solution containing phosphoric acid, which process, however, for reasons already stated, is not I A further object lies in the provision of an electrolytic polishing process which can be economically applied on a mass production scale whereby the apparatus, the time and labor costs, as compared with mechanical polishing, are substantially reduced.

A still further object is to provide such a process of polishing that leaves no deposit of grease or other foreign matter upon the article which cannot be readily removed by rinsing in water or the like.

Other objects and advantages of my invention will become more apparent from the following detailed description.

For purposes of illustration, I prefer to disclose my invention as it may be applied to the polishing of an article made of an alloy commonly known to those who are skilled in the art as yellow brass. However, it is to be clearly understood that my invention is applicable to a number of metals and metal alloys such as, for example, copper, zinc and their usual alloys which form solid solutions. Alloys containing only minute or insignificant quantities of other elements, such as lead, silicon, iron and the like which form precipitates, evolve gases or otherwise interfere with the process, may also be polished in accordance with my invention.

The electrolyte solution which I prefer to use in the polishing of a yellow brass article comprises a solution made according to the following proportions:

Parts by weight 4 Water (H2O) Chromic acid (CrOa) 12.5

Sodium dichromate (NazCrzOaZI-IzO) 37.5

Acetic acid (CHsCOOH) 12.5 Sulfuric acid (H2804) 10 Obviously, a solution having the same composition could be made from different substances such as, for example, by supplying the sulfate radical as sodium sulfate '(NazSO4) and by replacing some of the dichromate with chromic acid (CrOs).

The article to be polished constitutes th anode and is submerged in a bath of the electrolyte solution. If desired, a metallic tank, preferably steel or alloy steel, which constitutes the cathode may be used as a container for the electrolyte solution. necessarily constitute the cathode, inasmuch as separate cathodes may be used. The solution should be thoroughly agitated at all times during the polishing process and I prefer to accomplish the agitation by bubbling air through the solution, although other methods may of course be used, if desired. Polishing of the article occurs when an electrical current suillcient to provduce an average density of about 1000 amperes per square footon the anode surface is supplied to the circuit and the temperature of the electrolyte solution is kept fairly constant, preterably about 86 F. The time required to polish the article depends upon the condition'of its sur- However, the tank need not face before polishing. Articles'which have been rough polished on an emery wheel, for example, usually will require from two to five minutes of electrolytic polishing in order to obtain a mirrorlike surface. Pipe, stampings and forgings of comparatively smoother surfacing, may be polished in from one to five minutes, depending upon the initial condition of the surface to be polished.

From the foregoing, it is apparent that the physical and chemical conditions provided by my invention may be varied considerably without completely eliminating the desired polishing action. The amount of acetic acid per 100 parts (by weight) of water, for example, may be varied up to 200 parts, or if desired other soluble aliphatic acids, substituted aliphatic acids, organic or relatively weak inorganic acids which are not easily oxidized by the chromic acid, or salts of the above mentioned acids which lower and stabilize the hydrogen ion concentration of the solution may be used in place of the acetic acid. The total amount of chromate (both that which is supplied in the form of chromic acid and in the form of chromate salts) may be varied from 10 to 250 parts by weight per 100 parts of water. The amount of sulfuric acid may be varied from to 20 parts by weight per 100 parts of water. The proportion of chromate which may be added in the form of salts depends upon the quantities of acetic (or equivalent) and sulfuric acids present in the solution. Also, the temperature of the electrolyte may vary considerably, polishing having been accomplished at temperatures ranging from 50 F. to 167 F. The minimum electrical current density at the anode varies from 200 to 1000 amperes per square foot, depending upon the composition of the electrolyte. Satisfactory polishing of articles has been accomplished while using a current density as high as 3000 amperes per square foot.

It has been found that polishing can be effected in an aqueous solution of chromic acid alone, but the current density necessary is very high: in the case of brass, for example, about 5000 amperes per square foot is required. The addition of chromate salts makes polishing possible at lower current densities, but it is difficult to obtain good polishing over the whole of an irregular surface without the addition of the acetate radical or other buffering agents.

I believe that the polishing action of my process results from the removal of elevated portions or projections of the surface metal by the electrolytic dissolution of the latter in such a man- 5 ner that the surface is smoothed out and a,

lustrous, mirror-like finish is thereby imparted to the metal. Evidently a concentrated film of electrolyte, which has a higher electrical resistance than the bulk of the solution, is formed over the surface of the anode. The film is thicker above the scratches or depressions in the surface, hence these portions receive less current than the higher or unscratched portions of the surface and the tendency, therefore, is to dissolve the latter portions much more rapidly than the lower portions forming the scratches or depressions. Agitation of the solution prevents the anodic film from flowing over the surface and producing an uneven attack on the different areas thereof and at the same time prevents the formation of crystalline precipitates upon the surface. It appears that a strongly oxidizing electrolyte solution is necessary in order to keep the products of anodic dissolution in solution.

It is my theory that the acetic acid, or its equivalent, which constitutes an ingredient of the electrolyte solution serves the purpose of increasing the electrical resistance of the anodic film as has already been mentioned. The sulfate which I supply to the electrolyte improves the brightness of the polish and makes possible a better distribution of the polishing effect over irregular surfaces.

It is to be understood that other modes of applying the principle of my invention may be utilized, such changes being included as regards the ingredients herein stated which fall within the spirit and scope of the claim appended thereto. I claim: I

A method of surface polishing zinc, copper and their alloys, comprising subjecting the metal to electrolytic action as an anode in a bath having a current density of not less than 200 amperes per square foot of surface being polished, the said bath consisting of water 100 parts by weight, chromic acid about 12.5 parts by weight, sodium dichromate about 37.5 parts by weight, acetic acid about 12.5 parts by weight, and sulfuric acid 50 about 10 parts by weight.

ROBERT W. MANUEL. 

